What were Pip's first impression of his parents and where had he derived these from?

Pip never knew his parents. He was raised by his sister and her husband, the blacksmith Joe Gargery. Because he did not know his parents and had never seen even a drawing of either of them, he derived his impressions of them from the lettering on their tombstones. Because of the shapes of the letters on his father's tombstone, Pip determined that his father must have been square, stout and dark, "with curly black hair." Likewise, the inscription below this, which noted that "Georgiana Wife of the Above" was also buried in the grave, gave Pip a particular impression of his mother. He determined that she must have been freckled and rather sickly. Pip does not give any particular reason for these conclusions he has drawn—on the contrary, he seems to accept that they are childish notions born of little factual substance. There were also five small gravestones marking the last resting places of five of Pip's brothers, from which Pip determined that "they had all been born on their backs with their hands in their trouser pockets." In this determination, Dickens captures the sometimes inexplicable whimsy of the childish mind: there is no real reason for Pip to have drawn these conclusions, and as an adult he cannot really explain them, but nevertheless, these were his first impressions of his deceased family.

Since both of Pip's parents died before he could remember them, he derived his first impressions of them from their tombstones. The shape of the lettering of his father's tombstone caused Pip to imagine him as a "square, stout, dark man" with black hair. Because of the way “Also Georgiana Wife of the Above,” was carved on the gravestone, Pip thought of his mother as having freckles and being sickly. This shows how a child's mind, in the absence of information, will use whatever is at hand to fashion its ideas.
We learn too that Pip's parents died before photographs were taken, so he was not able to see what they looked like that way. Apparently his older sister, with whom he lived along with her husband Joe, never described his parents to him, leaving him to wander the marshy graveyard and conjure what ideas he could.